Practice Bulletin #226, Screening for Chromosomal Abnormalities
Genetic screening
1. Genetic Screening
Applications and counseling
Francesca Lai (Biology SL)
2. What is genetic screening?
• Genetic screening provides a way to predict whether or not a specific
phenotype will occur.
Methods of Genetic screening:
Determining abnormalities Screening Embryos
Karyotyping Amniocentesis
Polymerase Chain Preimplantation genetic
Reaction (PCR) diagnosis (PGD)
Restriction Fragment
Length Polymorphisms Chorionic villus sampling
(RFLPS) / Southern (CVS)
Blotting
3. Karyotyping
• Definition: “Karyotyping is the process of pairing and ordering all the
chromosomes of an organism, thus providing a genome-wide image of an
individual's chromosomes.”
• Karyotypes are prepared from mitotic cells which are frozen in metaphase.
• Characteristic structural features for each chromosome are revealed.
• Can reveal changes in chromosome numbers linked to conditions such as
Down’s syndrome.
• Careful analysis can show more subtle changes as chromosomal deletions,
duplications, translocations or inversions.
• Increasing use of karyotyping for diagnosis of specific birth defects and
genetic disorders.
4. Process of Karyotyping
• Blood, skin, tumors (for • Amniotic fluid or chorionic
identifying cancer) are some of villus specimens are used for
the tissue types used for prenatal diagnosis.
diagnosis.
5. Applications of Karyograms
• Now used to diagnose a wide range of chromosomal abnormalities in
humans.
• Examples
• Aneuploidy; A disease caused by the absence or addition of a
chromosome. (e.g. Down syndrome)
• Translocations; An exchange of chromosome parts.
• Researches use karyotypes to identify genes within critical regions of which
mis-expression would cause symptoms in patients.
6. Amniocentesis
• Definition: “Procedure used for prenatal genetic testing to obtain a sample of
amniotic fluid from a pregnant woman. A long sterile needle is inserted
through the abdominal wall into the amniotic sac to obtain the fluid.”
• Oldest procedure for obtaining DNA from an unborn child
• Usually performed between 15 - 18 weeks of pregnancy
• Despite improvements, still carries a risk of miscarriage or infection at a ratio
about 1:400
8. Chorionic villus sampling (CVS)
• An alternative method to amniocentesis
which is more modern but more risky.
• Definition: Procedure used for prenatal
genetic testing in which a small piece of
the chorion is removed from a pregnant
woman. A catheter is inserted through the
vagina and cervix into the uterus. Suction
is then applied to remove the sample.
• Performed between 10 - 12 weeks of
conception.
• Testing for abnormalities in Fetal
DNA:
• Can test for: the child’s sex, down
syndrome...etc.
• Fetal DNA can also be used for
karyotyping
9. Preimplantation Genetic
Diagnosis
• Definition: “A process which allows parents to have the option of detecting
potential defects in an embryo within days after conception.”
10. Preimplantation Genetic
Diagnosis (Cont’d)
• PGD depends on methods that are routinely used for in vitro
fertilization (IVF).
• The most common method:
• A blastomere (type of cell produced by division of the egg post-
fertilization), is removed from the embryo about 3 days after
fertilization. Once the blastomere is removed for analysis, it must
be transferred to the mother’s uterus within the next one or two
days.
• PGD has a significantly higher error rate than amniocentesis or
CVS; those two methods are often recommended to be used as
follow up procedures to PGD.
11. Pros & Cons of
genetic screening
PROS CONS
Prevention of the birth of babies with lethal
diseases (i.e. Tay-Sachs disease) “Designer babies”
which genes to screen: seriously harmful
Can determine ancestry genes vs. harmless genes.
(can we go too far?)
Economic and social consequences of the
Can predict forthcoming diseases (cancer)
test results
12. Bibliography
• O'Connor, Clare. "Karyotyping | Learn Science at Scitable." Nature Publishing
Group : Science Journals, Jobs, and Information. 2008. Web. 25 Feb. 2011.
<http://www.nature.com/scitable/topicpage/karyotyping-for-chromosomal-
abnormalities-298>.
• O'Connor, Clare. "Prenatal Screen Detects Fetal Abnormalities | Learn
Science at Scitable." Nature Publishing Group : Science Journals, Jobs, and
Information. 2008. Web. 25 Feb. 2011. <http://www.nature.com/scitable/
topicpage/Prenatal-Screen-Detects-Fetal-Abnormalities-PGD-and-306>.
• Wetzstein, Mike. "GENETIC SCREENING." NDSU - North Dakota State
University. 1996. Web. 25 Feb. 2011. <http://www.ndsu.edu/pubweb/~mcclean/
plsc431/students/mike.htm>.
Editor's Notes
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Add Chemical to stop mitosis in metaphase: The fixative is called colchicine, which poisons the mitotic spindle and causes the cells to become frozen in metaphase.\n\nAdd stain to enhance chromosomes: The stain is a hypotonic solution which causes the nuclei of the cells to swell and the cells to burst. \n
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A single cell, or blastomere, is removed from an embryo that has been fertilized in vitro (In an artificial environment outside the living organism. In this case, the embryo.), and this cell is tested for genetic abnormalities. If the blastomere does not appear to have the defect under investigation, the rest of the embryo can then be transferred to the mother's uterus.\n
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Ancestry: Used in forensic science. Unlike the traditional testing to detect gene mutations associated with disease, forensic genetic testing can identify crime victims, rule out suspects or establish biological relationships. Genealogical genetic testing allows one to trace their ancestry. It allows individuals to find out the probability that they are, or are not, related to another person within an estimated number of generations.\n\nPredict: Cancer researchers have discovered gene mutations that could increase a woman's risk of getting cancer. For example those who carry a particular mutation on the BRCA1 gene have an increased risk of breast cancer and genetic testing informs women if they carry this mutation. This kind of genetic testing is called predictive or pre-symptomatic testing.\n\nEconomic and social consequences: Genetic testing is often accompanied by genetic counseling because of the psychological problems that may accompany the results. There are ethical issues that also come up with genetic testing. The possibility of genetic discrimination in employment or insurance is also a concern. People may avoid genetic testing out of fear that it will affect their ability to obtain health or life insurance or find a job. \n\n
